I am going to be honest with you.
This game has stolen many, many hours of my life. I have changed the course of History in an infinite number of ways, and have played most nations, from Germany to the USSR to Cuba to Portugal. It is the best 20th century (until 1950, anyway) grand strategy game in existence. It is far better than Hearts of Iron III.
But what is just fantastic about this game is two things: the community and the mods.
The community is one of the greatest, best bunch of history maniacs and armchair generals I have ever seen. Going to the forums is a rare and wonderful experience. And they are exceedingly nice. And they will advise you and help you. And they will discuss intricate, microscopic details of historical events with you. And you will learn. And you will like it. And you will make friends. A fantastic bunch of people.
The mods, as if the game didn't have enough content already, are incredibly fun to play, and some are incredibly detailed as well. They are not that difficult to install, and you will have a ton of fun with them. I personnally recommend Kaiserreich (Germany wins WWI) and New World Order 2 (get to play during the Cold War as well, including alternative history events for a German victory in WWII). And, of course, the community has also dedicated itself to these mods, especially Kaiserreich.
I have been playing strategy games since Civilization (the first one), and this is the best strategy game I have ever played. Period. If you like strategy games, it is a must own.